prevent crash in send_error when fmt input argument is NULL - happens when user code sends a HTTP error response with just a HTTP response code without additional explanation (reason in fmt+args).
prevent crash in send_error when fmt input argument is NULL - happens when user code sends a HTTP error response with just a HTTP response code without additional explanation (reason in fmt+args).

commshare/civet-webserver,GerHobbelt/civet-webserver,commshare/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,commshare/civet-webserver,commshare/civet-webserver,commshare/civet-webserver,GerHobbelt/civet-webserver,commshare/civet-webserver,GerHobbelt/civet-webserver,commshare/civet-webserver